1. Field of the Invention
The present invention is in the field of infrared radiation and specifically relates to a device for physically interposing a number of filters into a beam of infrared radiation in a fixed sequence.
2. The Prior Art
The device of the present invention is expected to find its greatest application in nondispersive infrared gas analyzers in which the presence of particular gas components is tested by interposing several infrared radiation filters in a beam of infrared radiation. In a typical instrument, the filters have narrow bandpasses which are centered at absorption lines that are characteristic of the various components.
Because the characterizing absorption lines lie in the infrared portion of the electromagnetic spectrum, there is a likelihood that the results might be distorted by background infrared radiation emitted from surrounding objects at room temperature.
Fortunately, the background radiation is emitted continuously, which suggests that modulation of the test beam can be used to distinguish its radiation from the continuous radiation of the background.
Where only a single wavelength is involved, the use of spoked chopper wheels is well known. These wheels consist of alternating opaque and transparent areas, and are usually driven by an electrical motor. It is also known to put filters in the openings in the chopper wheel where multiple wavelength operation is desired. Motor-driven wheels have some disadvantages, such as the size and weight of the motor, power consumption, bearing wear, and mechanical vibration. It is also known that some of these disadvantages can be overcome by mounting the optical filters on a tuning fork or other structure driven at resonance by an electromagnet.
The vibrating devices are not without disadvantages. For example, the midpoint of the oscillation depends on the orientation of the apparatus with respect to gravity. Likewise, any acceleration experienced by the device will have a similar effect. Further, vibrating devices do not provide a constant linear velocity. Finally, if because of fluctuations in the driving current or variations in temperature, the amplitude of the vibration does not remain constant, the operation of the system could be impaired. Although the device of the present invention falls into the class of resonating instruments, the present invention includes means that prevent these problems from arising.
Certain aspects of the present invention are shown in the prior art, as disclosed below. However, it appears that the specific combination of features advantageously employed in the present invention is truly novel.
In U.S. Pat. No. 3,694,086, May shows the use of a piezoelectric transducer to selectively and cyclically introduce filters into an optical path. The filters are mounted on the end of a piezo crystal transducer, which lengthens and shortens in response to applied electrical signals. The device does not make use of mechanical resonance, and the amplitude of the vibration must be relatively small.
In U.S. Pat. No. 3,135,869, Rosenthal shows a mirror mount that employs a non-resonant piezoelectric mechanism for tilting a mirror about two orthogonal axes.
In U.S. Pat. No. 4,171,918, Mactaggart shows a tuning fork driven at resonance by an electromagnet for alternately introducing two filters of different wavelengths into the optical path. A similar system is shown by Dewey, Jr. in U.S. Pat. No. 3,853,407.
In U.S. Pat. No. 3,020,414, McKnight, et al. show a mirror scanning mechanism that employs electromagnets to tilt a torsionally-mounted mirror. In U.S. Pat. No. 3,071,036 McKnight, et al. show the use of five electromagnets to produce nutation of a mirror by sequential activation of the driving coils.
In U.S. Pat. No. 3,877,812, Thompson shows a clockwork type of device that uses a cam to generate the motion that changes from one filter to the next.
In U.S. Pat. No. 3,529,889, De Mey II shows a motor-driven non-resonant mechanism for sliding a desired filter into position.
In U.S. Pat. No. 3,194,962, Carlon, et al. show a filter wheel driven by an electric motor.
In U.S. Pat. No. 4,264,209, Brewster shows an electromagnetically driven pendulum on which are mounted two filters.
As will be seen below, the device of the present invention overcomes certain problems that were inherent in these prior art inventions to achieve a new and useful result.